Fin field effect transistor having angled fin sidewall

ABSTRACT

A fin field effect transistor (FinFET) includes a substrate and a fin having a first height over a surface of the substrate. The fin includes a first portion comprising a first sidewall, wherein the first sidewall is angled with respect to the surface of the substrate at a first angle. The fin further includes a second portion comprising a second sidewall, wherein the second sidewall is angled with respect to the surface of the substrate at a second angle, and the second angle is different from the first angle. The FinFET further includes a gate structure over the fin.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 14/933,120, filed Nov. 5, 2015, which is a divisional of U.S. application Ser. No. 14/102,644, filed Dec. 11, 2013, now U.S. Pat. No. 9,196,732, issued Nov. 24, 2015, which is a continuation of U.S. application Ser. No. 13/476,252, filed May 21, 2012, now U.S. Pat. No. 8,629,512, issued Jan. 14, 2014; which claims the priority of U.S. Provisional Application No. 61/616,965, filed Mar. 28, 2012, which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The disclosure relates to a fin field effect transistor, and a method of forming the same.

BACKGROUND

As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a fin field effect transistor (FinFET). A typical FinFET is fabricated with a thin vertical “fin” (or fin structure) extending from a substrate formed by, for example, etching away a portion of a silicon layer of the substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over (e.g., wrapping) the fin. Having a gate on both sides of the channel allows gate control of the channel from both sides. In addition, strained materials in recessed source/drain (S/D) portions of the FinFET utilizing selectively grown silicon germanium may be used to enhance carrier mobility.

However, there are challenges to implement such features and processes in complementary metal-oxide-semiconductor (CMOS) fabrication. As the spacing between vertical fins decreases, these problems are exacerbated. For example, the FinFET is not fully depleted if gate electrode does not fully wrap the channel of the FinFET, thereby increasing the likelihood of device instability and/or device failure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method of fabricating a gate stack of a Fin Field Effect Transistor (FinFET) according to various aspects of the present disclosure; and

FIGS. 2A-8C are perspective, top-down, side, and cross-sectional views of a FinFET comprising a gate stack at various stages of fabrication according to various embodiment of the present disclosure.

DESCRIPTION

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Referring to FIG. 1, illustrated is a flowchart of a method 100 of fabricating a gate stack of a fin field effect transistor (FinFET) according to various aspects of the present disclosure. The method 100 begins with step 102 in which a substrate is provided. The method 100 continues with step 104 in which a fin is formed in the substrate, wherein a base of an upper portion of the fin is broader than an apex of the upper portion, wherein the upper portion has first tapered sidewalls and a top surface. The method 100 continues with step 106 in which a gate dielectric covering the first tapered sidewalls and the top surface is formed. The method 100 continues with step 108 in which a conductive gate strip traversing over the gate dielectric is formed, wherein the conductive gate strip has second tapered sidewalls along a longitudinal direction of the fin. The discussion that follows illustrates embodiments of FinFETs that can be fabricated according to the method 100 of FIG. 1.

FIGS. 2A-8C are perspective, top-down, side, and cross-sectional views of a FinFET 200 comprising a tapered gate stack 230 at various stages of fabrication according to various embodiment of the present disclosure. As employed in the present disclosure, the FinFET 200 refers to any fin-based, multi-gate transistor. The FinFET 200 may be included in a microprocessor, memory cell, and/or other integrated circuit (IC). It is noted that, in some embodiments, the performance of the operations mentioned in FIG. 1 does not produce a completed FinFET 200. A completed FinFET 200 may be fabricated using complementary metal-oxide-semiconductor (CMOS) technology processing. Accordingly, it is understood that additional processes may be provided before, during, and/or after the method 100 of FIG. 1, and that some other processes may only be briefly described herein. Also, FIGS. 2A through 8C are simplified for a better understanding of the concepts of the present disclosure. For example, although the figures illustrate the FinFET 200, it is understood an integrated circuit (IC) may comprise a number of other devices comprising resistors, capacitors, inductors, fuses, etc.

Referring to FIGS. 2A and 2B, and step 102 in FIG. 1, a substrate 202 is provided. FIG. 2A is a perspective view of the FinFET 200 having a substrate 202 at one of the various stages of fabrication according to an embodiment, and FIG. 2B is a cross-sectional view of FinFET 200 taken along the line a-a of FIG. 2A. In at least one embodiment, the substrate 202 comprises a crystalline silicon substrate (e.g., wafer). The substrate 202 may comprise various doped regions depending on design requirements (e.g., p-type substrate or n-type substrate). In some embodiments, the doped regions may be doped with p-type or n-type dopants. For example, the doped regions may be doped with p-type dopants, such as boron or BF₂; n-type dopants, such as phosphorus or arsenic; and/or combinations thereof. The doped regions may be configured for an n-type FinFET, or alternatively configured for a p-type FinFET.

In some alternative embodiments, the substrate 202 may be made of some other suitable elemental semiconductor, such as diamond or germanium; a suitable compound semiconductor, such as gallium arsenide, silicon carbide, indium arsenide, or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. Further, the substrate 202 may include an epitaxial layer (epi-layer), may be strained for performance enhancement, and/or may include a silicon-on-insulator (SOI) structure.

In one embodiment, a pad layer 204 a and a mask layer 204 b are formed on the semiconductor substrate 202. The pad layer 204 a may be a thin film comprising silicon oxide formed, for example, using a thermal oxidation process. The pad layer 204 a may act as an adhesion layer between the semiconductor substrate 202 and mask layer 204 b. The pad layer 204 a may also act as an etch stop layer for etching the mask layer 204 b. In at least one embodiment, the mask layer 204 b is formed of silicon nitride, for example, using low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). The mask layer 204 b is used as a hard mask during subsequent photolithography processes. A photo-sensitive layer 206 is formed on the mask layer 204 b and is then patterned, forming openings 208 in the photo-sensitive layer 206.

Referring to FIGS. 3A, 3B, and 3C, and step 104 in FIG. 1, after formation of the openings 208 in the photo-sensitive layer 206, the structure in FIGS. 3A, 3B, and 3C is produced by forming a fin 212 in the substrate 202, wherein the base 214 b of an upper portion 214 of the fin 212 is broader than the apex 214 t, wherein the upper portion 214 has first tapered sidewalls 214 w and a top surface 214 s (shown in FIGS. 6B and 6C). FIG. 3A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment. FIG. 3B is a cross-sectional view of FinFET 200 taken along the line a-a of FIG. 3A and FIG. 3C is a top-down view of FinFET 200 of FIG. 3A.

The mask layer 204 b and pad layer 204 a are etched through openings 208 to expose underlying semiconductor substrate 202. The exposed semiconductor substrate 202 is then etched to form trenches 210 with a first surface 202 s of the semiconductor substrate 202. A portion of the semiconductor substrate 202 between trenches 210 forms one semiconductor fin 212. In the depicted embodiment, the semiconductor fin 212 comprises an upper portion 214 and a lower portion 216 (separated by the dashed line). In the present embodiment, the upper portion 214 and the lower portion 216 comprise the same material, such as silicon.

Trenches 210 may be strips (viewed from in the top of the FinFET 200) parallel to each other, and closely spaced with respect to each other. Trenches 210 each have a width, a depth, and are spaced apart from adjacent trenches by a spacing S. For example, the spacing S between trenches 210 may be smaller than about 30 nm. In an alternative embodiment, trenches 210 may be continuous and surrounding the semiconductor fin 212 (shown in FIG. 3C). The photo-sensitive layer 206 is then removed. Next, a cleaning may be performed to remove a native oxide of the semiconductor substrate 202. The cleaning may be performed using diluted hydrofluoric (DHF) acid.

Liner oxide (not shown) is then optionally formed in the trenches 210. In an embodiment, liner oxide may be a thermal oxide having a thickness ranging from about 20 Å to about 500 Å. In some embodiments, liner oxide may be formed using in-situ steam generation (ISSG) and the like. The formation of liner oxide rounds corners of the trenches 210, which reduces the electrical fields, and hence improves the performance of the resulting integrated circuit.

FIG. 4A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 4B is a cross-sectional view of FinFET 200 taken along the line a-a of FIG. 4A. Trenches 210 are filled with a dielectric material 218. The dielectric material 218 may include silicon oxide, and hence is also referred to as oxide 218 in the present disclosure. In some embodiments, other dielectric materials, such as silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), or a low-K dielectric material, may also be used. In an embodiment, the oxide 218 may be formed using a high-density-plasma (HDP) CVD process, using silane (SiH₄) and oxygen (O₂) as reacting precursors. In other embodiments, the oxide 218 may be formed using a sub-atmospheric CVD (SACVD) process or high aspect-ratio process (HARP), wherein process gases may comprise tetraethylorthosilicate (TEOS) and/or ozone (O₃). In yet other embodiment, the oxide 218 may be formed using a spin-on-dielectric (SOD) process, such as hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ).

FIGS. 4A and 4B depict the resulting structure after the deposition of the dielectric material 218. A chemical mechanical polish is then performed, followed by the removal of the mask layer 204 b and pad layer 204 a. The resulting structure is shown in FIGS. 5A and 5B. FIG. 5A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 5B is a cross-sectional view of FinFET 200 taken along the line a-a of FIG. 5A.

The remaining portions of the oxide 218 in the trenches 210 are hereinafter referred to as insulation regions 218 a. In double-gate embodiments, the mask layer 204 b and pad layer 204 a remain on the top of the fin 212 (not shown). In triple-gate embodiments, the mask layer 204 b is formed of silicon nitride, the mask layer 204 b may be removed using a wet process using hot H₃PO₄, while the pad layer 204 a may be removed using diluted HF acid, if formed of silicon oxide. The mask layer and pad layer remaining on top of the fin may prevent the top of the fin from turn-on to form a double-gate FinFET. In some alternative embodiments, the removal of the mask layer 204 b and pad layer 204 a may be performed after the recessing of the insulation regions 218 a, which recessing step is shown in FIGS. 6A, 6B, and 6C.

In an alternative embodiment, the upper portion 214 of the fin 212 is replaced by another semiconductor material to enhance device performance. Using insulation regions 218 a as a hard mask, the upper portion 214 of the fin 212 is recessed by an etching step. Then a different material such as Ge is epi-grown to fill the recessed portion. In the depicted embodiment, the upper portion 214 of the fin 212 such as Ge and lower portion 216 of the fin 212 such as Si comprise different materials.

As shown in FIGS. 6A, 6B, and 6C, after the removal of the mask layer 204 b and pad layer 204 a, the insulation regions 218 a are recessed by an etching step, resulting in recesses 220. FIG. 6A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment. FIG. 6B is a cross-sectional view of FinFET 200 taken along the line a-a of FIG. 6A and FIG. 6C is a top-down view of FinFET 200 of FIG. 6A. In one embodiment, the etching step may be performed using a wet etching process, for example, by dipping the substrate 202 in hydrofluoric acid (HF). In another embodiment, the etching step may be performed using a dry etching process, for example, the dry etching process may be performed using CHF₃ or BF₃ as etching gases.

The remaining insulation regions 218 b may be strips (viewed from in the top of the FinFET 200) parallel to each other, and closely spaced with respect to each other. In an alternative embodiment, the remaining insulation regions 218 b may be continuous and surrounding the semiconductor fin 212 (shown in FIG. 6C). FIG. 6C is top-down view of FinFET 200 of FIG. 6A and further comprises remaining insulation regions 218 b not shown in FIG. 6A. Further, the insulation regions 218 b cover a portion of the first surface 202 s, wherein a top of the insulation region defines a second surface 218 s.

In the depicted embodiment, the fin 212 through an opening in the insulation region 218 b to a first height H₁ above the second surface 218 s, wherein the base 214 b of an upper portion 214 (shown by the dashed line) of the fin 212 is broader than the apex 214 t, wherein the upper portion 214 has first tapered sidewalls 214 w and top surface 214 s (or defined as a third surface 214 s). In one embodiment, the base 214 b may be coplanar with the second surface 218 s, although the base 214 b may also be higher or lower than the second surface 218 s. The upper portion 214 of the fin 212 thus is used to form a channel region of the FinFET 200.

In at least one embodiment, an angle 214 a of the first tapered sidewalls 214 w to the first surface 202 s is from about 84 degrees to 88 degrees. In some embodiments, a difference between a maximum width W₂ of the first tapered sidewalls 214 w and a width W₁ of the third surface 214 s is in the range of about 1.5 nm to 5 nm. In some embodiments, a first height H₁ of upper portion 214 above the second surface 218 s is in the range of about 20 to 50 nm.

In some embodiments, the semiconductor fin 212 further comprises a lower portion 216 extending downward from the base 214 b to the first surface 202 s has a second height H₂. The lower portion 216 has third tapered sidewalls 216 w. In at least one embodiment, an angle 216 a of the third tapered sidewalls 216 w to the first surface 202 s is from about 60 degrees to 85 degrees. In some embodiments, a difference between a maximum width W₃ of the third tapered sidewalls 216 w and the maximum width W₂ of the first tapered sidewalls 214 w is in the range of about 3 nm to 10 nm. In yet another embodiment, a ratio of the first height H₁ to the second height H₂ is from about 0.2 to 0.5. Having more rigid volume than the upper portion 214, the lower portion 216 can avoid fin 212 deformation of the FinFET 200 due to high stress in the insulation regions 218 b.

A tapered gate stack 230 is then formed over the substrate 202 over the first tapered sidewalls 214 w and the third surface 214 s of the upper portion 214 and extending across the second surface 218 s of the insulation region 218 b. In some embodiments, the tapered gate stack 230 comprises a gate dielectric 222 b and a gate electrode layer 224 b over the gate dielectric layer 222 b (shown in FIGS. 8A, 8B, and 8C).

As depicted in FIGS. 7A and 7B, and step 106 in FIG. 1, for fabricating a gate stack (such as a tapered gate stack 230 shown in FIGS. 8A, 8B, and 8C), the structure in FIGS. 7A and 7B are produced by forming a gate dielectric 222 to cover the first tapered sidewalls 214 w and the third surface 214 s of the upper portion 214 and extending across the second surface 218 s of the insulation region 218 b. FIG. 7A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 7B is a cross-sectional view of FinFET 200 taken along the line a-a of FIG. 7A.

In some embodiments, the gate dielectric 222 may include silicon oxide, silicon nitride, silicon oxy-nitride, or high-k dielectrics. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and/or mixtures thereof. In the depicted embodiment, the gate dielectric 222 is a high-k dielectric layer with a thickness in the range of about 10 angstroms to 30 angstroms. The gate dielectric 222 may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation, UV-ozone oxidation, or combinations thereof. The gate dielectric 222 may further comprise an interfacial layer (not shown) to reduce damage between the gate dielectric 222 and upper portion 214 of the fin 212 (i.e., channel region of the FinFET 200). The interfacial layer may comprise silicon oxide.

Then, as depicted in FIGS. 7A and 7B, and step 108 in FIG. 1, the gate electrode layer 224 is formed over the gate dielectric 222. In the present embodiment, the gate electrode layer 224 covering the upper portion 214 of the semiconductor fin 212 is used to form a separate FinFET 200. In an alternative embodiment, the gate electrode layer 224 covers the upper portion 214 of more than one semiconductor fin 212 (not shown), so that the resulting FinFET comprises more than one fin.

In some embodiments, the gate electrode layer 224 may comprise a single-layer or multilayer structure. In at least one embodiment, the gate electrode layer 224 comprises poly-silicon. Further, the gate electrode layer 224 may be doped poly-silicon with uniform or non-uniform doping. In some embodiments, the gate electrode layer 224 comprises a metal selected from a group of W, Cu, Ti, Ag, Al, TiAl, TiAlN, TaC, TaCN, TaSiN, Mn, and Zr. In some embodiments, the gate electrode layer 224 comprises a metal selected from a group of TiN, WN, TaN, and Ru. In the depicted embodiment, the gate electrode layer 224 comprises a thickness in the range of about 30 nm to about 60 nm. The gate electrode layer 224 may be formed using a suitable process such as ALD, CVD, PVD, plating, or combinations thereof.

The process steps up to this point have provided the substrate 202 having the gate dielectric 222 covering the first tapered sidewalls 214 w and third surface 214 s of the upper portion 214, and the gate electrode layer 224 formed over the gate dielectric 222. In some embodiments, a layer of photoresist is formed over the gate electrode layer 224 by a suitable process, such as spin-on coating, and patterned to form a patterned photoresist feature 226 over the gate electrode layer 224 by a proper lithography patterning method. The patterned photoresist feature 226 can then be transferred using a dry etching process to the underlying layers (i.e., the gate dielectric 222 and gate electrode layer 224) to form a gate stack along the longitudinal direction of the fin 212. The patterned gate electrode layer is referred to a conductive gate strip. The conductive gate strip thus wraps a channel portion of the exposed upper portion 214 of the fin 212.

However, the conductive gate strip along the longitudinal direction of the fin 212 (with the first tapered sidewalls 214 w) is perpendicular to the first surface 202 s. As such, the first tapered sidewalls 214 w with a wider bottom are not fully wrapped by the conductive gate strip, resulting in a non-fully depleted fin when the FinFET is in on-state. This decreases drain-induced-barrier-lowering (DIBL) and increases sub-threshold leakage on a lower portion of the first tapered sidewalls 214 w, thereby degrading the device performance.

Accordingly, the processing discussed below with reference to FIGS. 8A, 8B, and 8C may etch the gate dielectric 222 and gate electrode layer 224 to form a tapered gate stack along the longitudinal direction of the fin 212 to fully wrap the wider bottom of the first tapered sidewalls 214 w. This can help to form a fully depleted fin when the FinFET is in on-state, thereby improving DIBL and sub-threshold leakage of the FinFET 200 and thus upgrading device performance.

As depicted in FIGS. 8A, 8B, and 8C, and step 108 in FIG. 1, for fabricating a tapered gate stack 230, the structure in FIGS. 8A, 8B, and 8C FIGS. are produced by forming a conductive gate strip 224 b traversing over the gate dielectric 222 b, wherein the conductive gate strip 224 a has second tapered sidewalls 224 w along the longitudinal direction of the fin 212. FIG. 8A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment. FIG. 8B is a cross-sectional view of FinFET 200 taken along the line a-a of FIG. 8A and FIG. 8C is a side view of FinFET 200 along a plane perpendicular to the line a-a of FIG. 8A.

As depicted in FIGS. 8A, 8B and 8C, the patterned photoresist feature 226 can then be transferred using a dry etching process to the underlying layers (i.e., the gate dielectric 222 and gate electrode layer 224) to form the tapered gate stack 230 along longitudinal direction of the fin 212. In at least one embodiment, in which the gate electrode layer 224 is poly-silicon, the step of the dry etch process is performed under a source power of about 650 to 800 W, a bias power of about 100 to 120 W, and a pressure of about 60 to 200 mTorr, using Cl₂, HBr and He as etching gases. The patterned photoresist feature 226 may be stripped thereafter.

In the depicted embodiment, a remaining gate dielectric 222 b covers the first tapered sidewalls 214 w and the third surface 214 s, while a remaining gate electrode layer 224 b (or referred as a conductive gate strip 224 b) traverses over the remaining gate dielectric 222 b, wherein the conductive gate strip 224 b has second tapered sidewalls 224 w along the longitudinal direction of the fin 212. In at least one embodiment, an angle 224 a of the second tapered sidewalls 224 w to the first surface 202 s is from about 85 degree to 88 degree. In some embodiments, a ratio of a maximum width W₅ of the second tapered sidewalls 224 w to a minimum width W₄ of the second tapered sidewalls 224 w is from 1.05 to 1.25. In some embodiments, the conductive gate strip 224 b further comprises a substantially vertical portion 224 c over the second tapered sidewalls 224 w.

In the depicted embodiment, the remaining gate dielectric 222 b and conductive gate strip 224 b are combined and referred as the tapered gate stack 230. The tapered gate stack 230 may wrap the wider bottom of the first tapered sidewalls 214 w. Thus, method 100 can help to form a fully depleted fin when the FinFET is in on-state, thereby improving DIBL and sub-threshold leakage of the FinFET 200 and thus upgrading device performance.

In the depicted embodiment, the tapered gate stack 230 is fabricated using a gate-first process. In an alternative embodiment, the tapered gate stack 230 may be fabricated using a gate-last process. In one embodiment, the gate-last process comprises forming an inter-layer dielectric (ILD) surrounding the dummy tapered gate stack 230, removing a dummy conductive gate strip to form a trench in the ILD, then fill the trench with a conductive gate strip. In some embodiments, the gate-last process comprises forming an ILD surrounding a dummy tapered gate stack, removing the dummy conductive gate strip 224 b and a dummy gate dielectric to form a trench in the ILD, then fill the trench with a gate dielectric and a conductive gate strip.

It is understood that the FinFET 200 may undergo further CMOS processes to form various features such as source/drain regions, contacts/vias, interconnect metal layers, dielectric layers, passivation layers, etc. It has been observed that a modified gate stack may wrap the wider bottom of the first tapered sidewalls 214 w to form a fully depleted fin when the FinFET is in on-state, thereby improving DIBL and sub-threshold leakage of the FinFET 200 and thus upgrading device performance.

One aspect of this description relates to a fin field effect transistor (FinFET). The FinFET includes a substrate and a fin having a first height over a surface of the substrate. The fin includes a first portion comprising a first sidewall, wherein the first sidewall is angled with respect to the surface of the substrate at a first angle. The fin further includes a second portion comprising a second sidewall, wherein the second sidewall is angled with respect to the surface of the substrate at a second angle, and the second angle is different from the first angle. The FinFET further includes a gate structure over the fin. In some embodiments, a ratio of a height of the second portion to a height of the first portion ranges from about 0.2 to about 0.5. In some embodiments, the first portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and a difference between the first width and the second width ranges from about 3 nanometers (nm) to about 10 nm. In some embodiments, the second portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and the different between the first width and the second width ranges from about 1.5 nm to about 5 nm. In some embodiments, the first angle ranges from about 60-degrees to about 85-degrees. In some embodiments, the second angle ranges from about 84-degrees to about 88-degrees.

Another aspect of this description relates to a fin field effect transistor (FinFET). The FinFET includes a substrate and a fin over a surface of the substrate. The fin includes a first portion comprising a first sidewall and a first height. The fin further includes a second portion comprising a second sidewall and a second height, wherein the second sidewall is angled with respect to the first sidewall, and a ratio of the second height to the first height ranges from about 0.2 to about 0.5. The FinFET further includes an isolation feature over the surface of the substrate, wherein the isolation feature contacts the first portion. The FinFET further includes a gate structure over the fin and over the isolation feature. The gate structure includes a gate dielectric layer over the isolation feature and the fin, wherein the gate dielectric layer contacts at least the second portion. The gate structure further includes a conductive strip over the gate dielectric layer. In some embodiments, the first portion comprises a first material, the second portion comprises a second material, and the first material is different from the second material. In some embodiments, the second portion is over the first portion. In some embodiments, the first sidewall is angled with respect to the surface of the substrate. In some embodiments, a height of the second portion above the isolation feature ranges from about 20 nanometers (nm) to about 50 nm. In some embodiments, the first sidewall is substantially linear. In some embodiments, the first portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and a difference between the first width and the second width ranges from about 3 nm to about 10 nm. In some embodiments, the second portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and the different between the first width and the second width ranges from about 1.5 nm to about 5 nm. In some embodiments, the fin further includes a rounded section at an interface between the first portion and the second portion.

Still another aspect of this description relates to a method of forming a fin field effect transistor (FinFET). The method includes etching a substrate to define a fin comprising a first material, wherein the fin has a first height above a bottom surface of the substrate. The fin includes a first portion comprising first sidewalls tapered at a first angle. The fin further includes a second portion comprising second sidewalls tapered at a second angle different from the first angle. The method further includes depositing an insulating material over the substrate, wherein the deposited insulating material covers a top surface of the fin. The method further includes recessing the insulating material to expose at least the second portion of the fin. The method further includes forming a gate structure over the fin. The forming of the gate structure includes depositing a gate dielectric over the fin and over the recessed insulating material. The forming of the gate structure further includes depositing a conductive material over the gate dielectric. In some embodiments, the method further includes forming a liner oxide on the fin, wherein the forming of the liner oxide includes rounding corners of the fin. In some embodiments, the method further includes replacing a section of the fin with a second material different from the first material. In some embodiments, the replacing of the second of the fin includes etching the fin to define a recess; and epitaxially growing the second material to fill the recess. In some embodiments, the etching of the fin includes using the insulating material as a hard mask.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A fin field effect transistor (FinFET) comprising: a substrate; a fin having a first height over a surface of the substrate, wherein the fin comprises: a first portion comprising a first sidewall, wherein the first sidewall is angled with respect to the surface of the substrate at a first angle, and the first portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and a difference between the first width and the second width ranges from about 3 nanometers (nm) to about 10 nm; and a second portion comprising a second sidewall, wherein the second sidewall is angled with respect to the surface of the substrate at a second angle, and the second angle is different from the first angle; and a gate structure over the fin.
 2. The FinFET of claim 1, wherein a ratio of a height of the second portion to a height of the first portion ranges from about 0.2 to about 0.5.
 3. The FinFET of claim 1, wherein the second portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and the different between the first width and the second width ranges from about 1.5 nm to about 5 nm.
 4. The FinFET of claim 1, wherein the first angle ranges from about 60-degrees to about 85-degrees.
 5. The FinFET of claim 1, wherein the second angle ranges from about 84-degrees to about 88-degrees.
 6. A fin field effect transistor (FinFET) comprising: a substrate; a fin over a surface of the substrate, wherein the fin comprises: a first portion comprising a first sidewall and a first height; and a second portion comprising a second sidewall and a second height, wherein the second sidewall is angled with respect to the first sidewall, and a ratio of the second height to the first height ranges from about 0.2 to about 0.5; an isolation feature over the surface of the substrate, wherein the isolation feature contacts the first portion; and a gate structure over the fin and over the isolation feature, wherein the gate structure comprises: a gate dielectric layer over the isolation feature and the fin, wherein the gate dielectric layer contacts at least the second portion; and a conductive strip over the gate dielectric layer.
 7. The FinFET of claim 6, wherein the first portion comprises a first material, the second portion comprises a second material, and the first material is different from the second material.
 8. The FinFET of claim 6, wherein the second portion is over the first portion.
 9. The FinFET of claim 6, wherein the first sidewall is angled with respect to the surface of the substrate.
 10. The FinFET of claim 6, wherein a height of the second portion above the isolation feature ranges from about 20 nanometers (nm) to about 50 nm.
 11. The FinFET of claim 6, wherein the first sidewall is substantially linear.
 12. The FinFET of claim 6, wherein the first portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and a difference between the first width and the second width ranges from about 3 nm to about 10 nm.
 13. The FinFET of claim 6, wherein the second portion has a first width closest to the surface of the substrate and a second width farthest from the surface of the substrate, and the different between the first width and the second width ranges from about 1.5 nm to about 5 nm.
 14. The FinFET of claim 6, wherein the fin further comprises a rounded section at an interface between the first portion and the second portion.
 15. A fin field effect transistor (FinFET) comprising: a substrate; a fin over a surface of the substrate, wherein the fin comprises: a first portion comprising a first sidewall and a first height; and a second portion comprising a second sidewall and a second height, wherein the second sidewall is angled with respect to the first sidewall, and the first height is greater than the second height; an isolation feature over the surface of the substrate, wherein the isolation feature directly contacts the first portion; and a gate structure over the fin and over the isolation feature, wherein the gate structure directly contacts the second portion, and a dimension of the gate structure in a direction parallel to a top surface of the substrate is non-uniform.
 16. The FinFET of claim 15, an angle between the second sidewall and the surface of the substrate is closer to perpendicular than an angle between the first sidewall and the surface of the substrate.
 17. The FinFET of claim 15, wherein the isolation feature completely covers the first sidewall.
 18. The FinFET of claim 15, wherein the first portion directly contacts the second portion.
 19. The FinFET of claim 1, further comprising an isolation feature directly contacting the first portion, wherein the gate structure exposes an area of the isolation feature directly contacting the first portion.
 20. The FinFET of claim 15, a ratio of the second height to the first height ranges from about 0.2 to about 0.5. 